1. Field of Invention
The invention relates to the field of fabrication of airfoil structures and in particular to a method of fabricating rib structures of different sizes for an airfoil.
2. Description of Prior Art
A conventional airfoil usually has as its primary structure two or more spars extending spanwise along the airfoil spaced apart by a plurality of rib structures. Leading and trailing edge secondary structures are attached to this primary structure and all are enclosed by upper and lower wing skins. Additionally, mechanized control surfaces such as flaps, ailerons, etc., are incorporated. A typical example of an airfoil design is disclosed in U.S. Pat. No. 2,427,853, Aircraft Construction by E. R. Goodlett.
Past attempts to reduce the cost of fabricating airfoils by minimizing the amount of tooling required have not been particularly successful. For example, U.S. Pat. No. 2,360,433, Structural Member by P. M. Magruder uses two identical rib web halves which are overlapped on their bottom edges different amounts and joined together. Thus the capability of making rib structures for airfoils of tapering height is provided. A rib structure, made by using two such rib web halves, one inverted from the other, and assembled with a lap joint, as Magruder proposes, requires that one rib cap be facing inboard and the other outboard. While the airfoil can be made to taper in thickness, one of the rib caps, whether having an acute or obtuse angle relative to the rib web, will not be properly aligned with the taper angle of the airfoil since the rib web halves are inverted in forming the rib structures. Thus rib web halves with different rib cap angles will be necessary if they are to conform to the taper angle of the airfoil. This will require two separate fabrication tools.
In a second embodiment Magruder discloses a method of making rib structures that vary both in height and chord (length). This is accomplished by making the rib structure from four quarter pieces and overlapping their ends and lower edges. This four-piece rib structure has the disadvantages previously mentioned and furthermore can only provide a smooth aerodynamic surface for one rib structure position in the spanwise direction. If the length is shortened, to produce a rib structure of reduced chord, an aerodynamically unacceptable sharp peak is formed at the overlapping point on the upper surface of the airfoil. Conversely, if the chord is lengthened, there is an unacceptable indentation created at this point.
Therefore, it is a primary object of this invention to provide a method of fabricating structures for an airfoil at a lower cost by minimizing the number of tools required.
Another object of this invention is to provide a method of fabricating rib structures of varying height for an airfoil that is spanwise tapered in thickness with one rib fabrication tool.
A still further object of this invention is to provide a method of making rib structures of varying height and chord for an airfoil that both tapers in thickness and chord.